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This change updates the existing web platform tests for WebUSB to use
multi-global tests to test the API in the supported contexts. The
change updates the following tests:
* idlharness
* usb
* usbConnectionEvent

Bug: 841510
Change-Id: I6b118aa4121109cd6a48054c2ebc69b831660afc
Reviewed-on: https://chromium-review.googlesource.com/1087209Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Ovidio Henriquez <odejesush@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564981}